The combination of microelectronic circuits and low power transparent displays, such as liquid crystal displays ("LCDs"), has led to a wide variety of portable electronic products. "Transparent displays," as used herein, refers to visual display devices wherein either the visual message portion of the display or the background portion of the display is transparent or translucent. These products range from electronic watches to hand-held television receivers and laptop computers. Low power consumption is a critical requirement for each of these products.
Despite their considerable utility in conjunction with integrated circuits, LCDs have a number of shortcomings. In typical LCDs, the activated portion (perhaps representing a visual message) is darkened and the inactivated portion (perhaps representing a background for the visual message) is lightened. One shortcoming of LCDs is that the contrast between the activated and inactivated portions is relatively low, making the LCD hard to read. The contrast can be increased by backlighting the LCD, thereby producing a sharp contrast between the portions of the LCD darkened by activation and the light shining through the transparent, inactive portions. Currently, electroluminescence ("EL") devices provide the highest spatial and angular backlighting uniformity. EL devices, however, require a special, relatively high-voltage inverter and suffer from a relatively short lifetime. All conventional backlighting arrangements, including EL devices, are inefficient. Even in so complex an electronic structure as a portable computer, the power used in backlighting is responsible for a major portion of total battery drain.
U.S. Pat. No. 5,245,454, issued on Sep. 14, 1993, to Blonder, entitled "LCD Display With Microtextured Back Reflector And Method For Making Same," commonly assigned with the present invention and incorporated herein by reference, discloses a microtextured reflector having dispersion characteristics of enhanced uniformity and a display device that can be read over an enhanced range of viewing angles. Specifically, a reflector comprises a base plane having a microtextured surface comprising a pattern of microelements having maximum lateral dimensions less than 125 microns and smooth, continuous, mold-formed surfaces arising from the base plane. A preferred reflecting surface is composed of closely-packed, smooth-surfaced spherical segments having diameters of 25 microns and altitudes of 4 microns.
U.S. Pat. No. 5,349,503, issued on Sep. 20, 1994, to Blonder, et al., entitled "Illuminated Transparent Display With Microtextured Back Reflector," commonly assigned with the present invention and incorporated herein by reference, discloses a particularly efficient illuminating device for a transparent display comprising a microtextured reflecting surface formed on a major surface of a transparent substrate and a light source for directing light into the transparent substrate approximately parallel to the major surface. Preferably, the substrate has two major surfaces separated by a thickness in the range of 0.5 mm to 2.0 mm so that the substrate behaves as a light guide. A substantial portion of the light passing into the substrate undergoes total internal reflection until it hits a microelement and is reflected out of the substrate. The result is an efficient illuminating device particularly useful for backlighting LCDs or other transparent displays.
Accordingly, what is needed in the art is an illuminating device for a transparent display that provides light of acceptable spatial and angular uniformity with minimal power consumption and a method of manufacture therefor.